The spindle is intended to use angular contact bearing on top and bottom. I only had one left for testing so I put it on the bottom. I use a big 2.2kW VFD based water cooled spindle on my 4x4 router. The best part is how quiet it is. I cool it similar to the laser tube (pump and bucket). The water never even gets warm.

I think my new little spindle will work just fine for want it want. Time will tell how long the bearings last so I guess I cannot release the design until the end of time.

This is a big step up from the Low Cost Spindle I've been looking at from MIT. Would the second angle contact bearing be used to counter-act forces pulling down on the shank, or have I totally missed the point with the paired bearing concept? Is preloading an issue in the radial, axial or both directions?

I'm wonder where on the cost/benifit curve this design will land. I'm assuming somewhere b/t a rotozip and one of the wiz-bangs mentioned above. For me, it's all about pushing the 80/20 rule and of course the joy of tinkering.

As it sits with one AC (Angular Contact) bearing the MIT one is better.

Angular contact bearings are used in a minimum of pairs, sometimes triples if there is an extreme amount of force in one direction. Unlike a standard radial ball bearing where the load force is intended to be towards the side of the shaft or a thrust bearing where the load is in line with the shaft the races of an AC bearing have the ball contact at and angle, usually somewhere from 15 to 25 degrees from radial, that acts to take up thrust and radial load. To remove possible run out the bearings are preloaded so there is pressure on the balls. The amount of pressure depends on a lot of things, RPM, load, lubrication, etc. Often AC bearings will come in a matched pair that will have the inner races ground so when they are assembled and the races are locked together they will have the preload set.

I've got a fairly beefy 'rotozip' type tool from sears and I've ordered a shank from HK, so in about 2 months I'll be able to stage a spindle-off. With regard to the bearings; a little light reading leads me to conclude: 1) AC bearings are preloaded in the radial direction (ie the inner race is a little big and pushes outwardly) 2) an AC bearing only supports thrust from one direction, which is why they're usually paired.

The spindle is being tested by Edward Ford of ShapeOko. He has a few hours on it I think, but busy with a big batch of ShapeOkos. He said it is 12dB (by iPhone app) quieter than his default dremel spindle.